Browsing by Subject "Atomic Force Microscopy"

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Open Access
Construction of an atomic force microscope operating in air and liquid
(2003) Karaköse, Mustafa
In this thesis, a new atomic force microscope capable of measuring in liquid and air has been introduced. The highly improved design of the instrument eliminates some of the technical problems which are occurred before. The instrument makes use of the small amplitude AC-Mode technique to detect the interaction forces between the tip and the surface. Some results of initial test scans of the instrument are displayed.
Open Access
Enhanced higher-harmonic imaging in tapping-mode atomic force microscopy
(AIP, 2005) Balantekin, M.; Atalar, Abdullah
Higher-harmonics generation in a tapping-mode atomic force microscope is a consequence of the nonlinear tip-sample interaction force. The higher harmonics contain important information about the materials’ nanomechanical properties. These harmonics can be significantly enhanced by driving the cantilever close to a submultiple of its resonant frequency. We present the results of enhanced higher-harmonic imaging experiments on several samples. The results indicate that enhanced higher harmonics can be utilized effectively for both material characterization and surface roughness analysis with a high signal-to-noise ratio. © 2005 American Institute of Physics
Open Access
Force spectroscopy using bimodal atomic force microscopy
(2010) Aksoy, Mehmet Deniz
In atomic force microscopy (AFM) achieving compositional contrast while mapping topographical features is a challenging task. Conventional single mode frequency and amplitude modulation AFM techniques are sensitive to the properties of the tip sample interaction, however in the absence of additional information channels, compositional features such as elasticity and density cannot be distinguished from topographical variations. To tackle this problem bimodal excitation techniques are introduced. In bimodal amplitude modulation AFM, sensitivity to compositional features improves by recording the phase of the higher order vibrations, while the topography is acquired using the amplitude of the first order vibrations. Increased sensitivity to mechanical properties allows imaging delicate samples such as organic molecules using gentle forces. In this thesis we propose a force spectroscopy technique in which two modes of a cantilever are excited in such a way that the amplitudes of the components of the vibration stay constant. Presence of the force field modulates the properties of the primarily bi-harmonic vibration of the cantilever, which is, in our case, the instantaneous frequencies of vibration modes. The frequency shift of the first mode remains sensitive to topographical variation, whereas the frequency shift of the higher mode samples the gradient of the tip sample forces and allows us to extract the tip sample interaction as a function of separation within a single cycle of the slow oscillation. We provide an analytic treatment of the proposed scheme and confirm our predictions by numerical simulations. We present an analysis of the sensitivity of higher mode frequency shifts to compositional features in the presence of thermal and sensor noise. We demonstrate that the method is suitable for the fast acquisition of contact properties, especially in vacuum environment where the large quality factor of the cantilever limits the available bandwidth of the amplitude modulation techniques. Finally we investigate phase shifts in bimodal amplitude modulation AFM using the developed formalism and show that phase contrast can be optimized by solving a simpler problem in single mode amplitude modulation AFM.
Open Access
Genetically-tunable morphology and mechanical properties of bacterial functional amyloid nanofibers
(2017-05) Abdelwahab, Mohammad Tarek Hamed
The highly dynamic behavior of material systems exploited by nature results in research efforts to employ them as next generation biomaterials exhibiting sustainability and resistance against harsh environmental conditions. In recent years, functional protein-based structures started to be investigated heavily. Among these, bacterial biofilms present themselves as highly organized, hierarchical, dynamic material systems comprising cells, various carbohydrates, and extracellular proteins. They are known to be resistant against different kinds of disruptions by chemical, physical, and biological agents. These fascinating qualities make them potential candidates for next generation biomaterials. Motivated as above, we present in this M.S. thesis a comprehensive study of the morphological and mechanical properties (in terms of Young’s modulus) of biofilm structures assembled from bacterial amyloid nanofibers of Escherichia coli (E. coli) via imaging and force spectroscopy performed by the atomic force microscope (AFM). We used techniques adopted from genetic engineering to employ different E. coli mutants, allowing comparisons of Young’s modulus and morphology of different biofilm nanofibers based on genetic composition. In particular, we tuned the genetic expression of the major (CsgA) and minor (CsgB) proteins constituting bacterial amyloid nanofibers, with the optional addition of certain amino acid tags in order to have multiple controlled versions of biofilm nanofibers. After sample preparation, we calibrated a single AFM probe to be used for all experiments, and performed contact-mode imaging measurements to probe the morphological differences among these genetically-different biofilm amyloid nanofibers. In addition, we conducted nanoindentation experiments to obtain force spectroscopy curves. A precise processing routine was developed to extract the mechanical stiffness from acquired data. Furthermore, we statistically contrasted the stiffness values to reveal the genetic dependence of the mechanical properties of the final protein assembly. The processing routine was also able to detect the effect of the substrate on mechanical stiffness measurements. The experimental results presented in this thesis pave the road for the use of genetic engineering to rationally tune the mechanical as well as the morphological properties of bacterial amyloid nanofibers, and thereby underline the critical role that they may play as new generation biomaterials.
Open Access
Investigation of structural lubricity on platinum nanoparticles under ambient conditions
(2017-05) Özoğul, Alper
Structural lubricity describes a state of ultra-low friction involving relative motion between atomically flat and molecularly clean surfaces with incommensurate structures. While the occurrence of structural lubricity was quantitatively confirmed under ultrahigh vacuum (UHV) conditions first, recently reported experiments have demonstrated that structurally lubric sliding is achievable under ambient conditions as well, specifically at mesoscopic interfaces formed between thermally deposited gold nanoparticles and graphite. The question that is covered in this thesis is whether the observation of structural lubricity under ambient conditions is limited solely to gold nanoparticles. To answer this question, an investigation of the frictional behavior of platinum nanoparticles laterally manipulated on graphite has been conducted. In particular, platinum nanoparticles have been prepared by e-beam evaporation of a thin film of platinum on graphite, followed by post-deposition annealing. Morphological characterization of the particles was performed via scanning electron microscopy (SEM) and transmission electron microscopy (TEM), revealing a crystalline structure. X-Ray photoelectron spectroscopy (XPS) revealed no significant change in the electronic structure of platinum upon exposure to air, ruling out the oxidation of platinum nanoparticles under ambient conditions. This finding was supported by cross-sectional TEM measurements demonstrating the absence of an oxide layer on the particle surfaces. Lateral manipulation experiments have been performed on platinum nanoparticles of mesoscopic dimensions (4000 − 75,000 nm2) under ambient conditions via atomic force microscopy (AFM), whereby results indicated the occurrence of structural lubricity, although with a higher magnitude of friction forces when compared with gold nanoparticles. Thus, it is confirmed that the occurrence of structural lubricity between incommensurate, atomically flat surfaces should be independent of material choice, as stated in the theory. Lastly, an attempt has been made to alter the structure and chemistry of the sliding surface of platinum nanoparticles. For this purpose, platinum nanoparticles have been oxidised in a reactive oxygen plasma atmosphere. XPS results confirmed the existence of oxidised platinum, and structural characterization performed by SEM showed that there was no significant change in morphology. Lateral manipulation experiments performed on oxidised platinum nanoparticles showed that these nanoparticles experience approximately two times as much friction as platinum nanoparticles. The potential reasons behind this observation are discussed.
Open Access
MOCVD growth and optical properties of non-polar (11-20) a-plane GaN on (10-12) r-plane sapphire substrate
(Elsevier, 2010-11-15) Yu, H.; Ozturk, M.; Demirel, P.; Cakmak, H.; Özbay, Ekmel
Non-polar a-plane GaN film with crystalline quality and anisotropy improvement is grown by use of high temperature AlN/AlGaN buffer, which is directly deposited on r-plane sapphire by pulse flows. Compared to the a-plane GaN grown on AIN buffer, X-ray rocking curve analysis reveals a remarkable reduction in the full width at half maximum, both on-axis and off-axis. Atomic force microscopy image exhibits a fully coalesced pit-free surface morphology with low root-mean-square roughness (similar to 1.5 nm). Photoluminescence is carried out on the a-plane GaN grown on r-plane sapphire. It is found that, at low temperature, the dominant emission at similar to 3.42 eV is composed of two separate peaks with different characteristics, which provide explanations for the controversial attributions of this peak in previous studies. (C) 2010 Elsevier B.V. All rights reserved.
Open Access
Noise analysis of interdigital cantilevers for atomic force microscopy
(1998) Yaralıoğlu, G. Göksenin
Atomic force microscoiDe (AFM) is proved to be a powerful tool for atomic resolution surface imaging. The most crucial parts of an AFM system are the cantilever with an integrated tip and the deflection detection sensor. AFM systems measure deflections that are comparable to atomic dimensions using technicpies such as tunneling, interferometry, piezoresistive sensing and optical lever detection. Interdigital (ID) cantilevers are the most recently introduced method which makes use of its interferometric nature to improve deflection detection sensitivity. Basicallj^ ID cantilever is composed of two sets of interleaving fingers which create an optical phase grating. In this thesis, a detailed analysis of ID cantilevers will be presented. The theory underlying the o[)eration of the phase gratings with the response curves curd confirming e.xperimental results will be formulated. The noise performance of the ID cantilever will be compared to the optical lever detection method. We will present a new method for the mechaniccd noise calculation by using the analogy between electrical circuits and mechanical structures. This new method will be applied to the AFM cantilevers to calculate the noise correlation on the cantilever surface. We will also present the signal to noise ratio (SNR) calculation method on the cantilever. One of the basic problem of the all AF'M systems is the speed limitation due to single AF'M tip scanning at relatively low frequencies yielding low throughput. A direct approach to this problem is the operation of cantilever arrays instead of one cantilever. In this thesis, we will also present the electronics for cantilever arrays which increases the throughput of the AFM systems.
Open Access
Scanning hall probe microscopy (SHPM) using quartz crystal AFM feedback
(2005) Ürkmen, Koray
Scanning Hall Probe Microscopy (SHPM) is a quantitative and non-invasive technique for imaging localized surface magnetic field fluctuations such as ferromagnetic domains with high spatial and magnetic field resolution of ~50nm & 7mG/ Hz at room temperature. In the SHPM technique, Scanning Tunneling Microscope (STM) or Atomic Force Microscope (AFM) feedback is usually used for bringing the Hall sensor into close proximity of the sample. In the latter, the Hall probe has to be integrated with an AFM cantilever in a complicated microfabrication process. In this work, we have eliminated the difficult cantileverHall probe integration process; a Hall sensor is simply glued at the end of Quartz crystals, which are used as a force sensor. The sensor assembly is dithered at the resonance frequency and the quartz force sensor output is detected with a Lock-in and PLL system. SHPM electronics is modified to detect AFM topography and the phase, along with the magnetic field image. NIST MIRS (Magnetic Referance Sample) (Hard Disk) sample, 100 MB high capacity zip disk and Garnet sample are imaged with the Quartz Crystal AFM feedback and the performance is found to be comparable with the SHPM using STM feedback. Quartz Crystal AFM feedback offers a very simple sensor fabrication and operation in SHPM. This method eliminates the necessity of conducting samples for SHPM.
Open Access
Self-assembly of peptide nanofibers and their mechanical properties
(2012) Erkal, Turan Selman
Peptide nanofibers have been drawing attention because of their versatile, tailorable and functional properties in various research areas. The self-assembly mechanism of peptides and peptide amphiphile molecules is generally based on noncovalent interactions like hydrophobic, electrostatic and metal-ligand interactions. In this thesis, I investigated hydrophobic interaction of peptide amphiphiles (PAs) with other hydrophobic molecules and effect of pH change on self-assembly mechanism. The zinc phthalocyanine molecule was used as a hydrophobic probe to be encapsulated by peptide amphiphile molecules, which help to dissolve the molecule in water instead of an organic solvent. Charge neutralization of PAs by pH change led to nanofiber formation, which resulted in encapsulation and organization of zinc phthalocyanine molecules. The degree of self-assembly by pH change determined non-linear optical properties of zinc phthalocyanine molecule. Besides, morphological, mechanical and spectroscopic properties of phthalocyanine containing peptide nanofibers were characterized by TEM, SEM, oscillatory rheology, UV-Vis, fluorescence and circular dichroism spectroscopy. The mechanical properties of peptide and PA hydrogels and nanofibers have an essential place to determine applicability in different areas. Especially, PA and peptide molecules have been widely used in regenerative medicine studies and the stiffness of the extracellular matrix has a significant role on cellular behavior. In this thesis, viscoelastic properties of the peptide and PA gels were studied by oscillatory rheology. In addition to characterization of bulk mechanical properties of peptide gels, adhesion and stiffness of peptide nanofibers were determined by Atomic Force Microscopy.